1. Field of the Invention
The present invention relates to a servo demodulating circuit and method of a magnetic transfer medium.
2. Description of the Related Art
The conventional method of writing servo information (servo patterns) of a magnetic disk onto a disk operates by using a servo track writer (STW). In such a method, for example, the servo write time for a 20 Gb/disk-class magnetic disk requires approximately 10 minutes/disk. Since there has been a current trend of ever increasing track recording density in the magnetic disks, the time required for servo writing by STW has become longer, causing significant cost problems.
In light of the problems with STW, a method has been proposed as a new technique for recording servo information in patterns in a master disk and storing the information by magnetic transfer.
An example of the magnetic transfer method is, as described in Non-patent Document 1, a method for forming a magnetic film by making a fine asperity pattern on the master disk and generating a pattern by magnetic transfer. The time required for magnetic transfer to one magnetic disk is far shorter than the execution time of STW by the conventional method.
For commercial production of patterned media and discrete track media, due to production cost concerns and writing time length, it is difficult to perform electron beam lithography on every disk. Therefore, it is more realistic to manufacture these media via molds, such as nanoimprinting, which is suitable to a desired pattern.
FIG. 1 shows a heating nanoimprinting process relating to the prior technology. Specifically, a medium is produced by the steps below: (a) increasing the temperature of a disk on which a magnetic film is formed by coating the disk with resin such as polymethylmethacrylate (PMMA) resin by, for example, a spin coating method, (b) attaching and pressing a mold, prepared in advance and generated by electron beam lithography (EM lithography) against a disk by using a nanoimprinting apparatus and softening the resin coating through the application of heat, (c) cooling, afterwards, to room temperature and demolding the disk (detaching the disk from the mold), (d) etching the excessive resin on the disk with oxygen gas using a reactive ion etching (RIE) apparatus, (e) further etching the magnetic film by gas, and finally (f) removing the mask and attaching a protection film. During the process of etching the magnetic film, in order to preserve the etching's stability, etching via intermediate layer rather than via the PMMA resin is also proposed.
[Non-Patent Document 1]
“Magnetic Printing Technology”, Fuji Electric Journal, vol. 75, No. 3, 2002, pp. 173-177
In a conventional method using STW, the signal amplitude of the recorded servo pattern is uniform. In the magnetic transfer method described in the above Non-Patent Document, however, the extent of attachment between the master disk and a slave medium (magnetic spacing) directly influences the intensity of magnetic attraction (measure of magnetization), and thus several nanometers (nm) of difference in the height of asperity on the master disk or minute surface roughness of the slave medium would cause fluctuation of the replayed signal amplitude, creating a problem involving insufficient signal quality.
When demodulating a servo signal, a system correcting the sensitivity of the head or the sensitivity of the medium by auto gain control (AGC) or a system absorbing the fluctuation of the signal amplitude caused by the difference in recording density between the inner circumference and the outer circumference of the disk is employed. There is a problem, however, such that in a case of a magnetically transferred signal, if the above amplitude fluctuation is present in a sector, all of the fluctuation will result in demodulation errors.
A further problem exists such that when performing, in the conventional method, the magnetic transfer to the perpendicular magnetic recording medium, magnetization is not likely to be even, causing a significant demodulation error.
When employing the above nanoimprinting method, the edge of the PMMA resin may be chipped in the demolding process illustrated in FIG. 1C, or the convex portion of the mold may have a concentrated electrical field, creating variations in the edges in magnetic film etching process illustrated in FIG. 1E. The occurrence of such events causes a problem in which the edge positions are misaligned, resulting in phase fluctuation caused by amplitude fluctuation of the replayed signal.